It’s time to replace the fleet, and at about $3.1 million per car, the transit agency will need to come up with about $2.1 billion to do it, Bay Area Rapid Transit staff told agency directors Thursday.

If cars are breaking down, the trains are late. If the trains are late, people won’t ride the system, said agency spokesman Linton Johnson.

“This is about being on time,” he said. “If you can’t rely on BART, then why ride?”

BART has about 95 percent passenger on-time record. But that could drop to 80 percent or worse if the cars continue to decline in the next decade.

Thursday’s briefing is only the beginning of what will be a long process to design and buy the new cars, Linton said.

Commuters won’t see the new cars for at least six years or so.

The new $3.1 million cars will have to fit the unique width in between the rails, requiring a custom design. Officials are hoping to add more space for strollers, wheelchairs, bikes and standing customers.

Staff are also floating the idea of three doors on each car instead of two.

As always, the big question will be how to pay for the purchase.

In 1971, the 669 trains cost $250 million, funded by the property taxes that built the system.

The Metropolitan Transportation Commission is looking to set aside about $20 million to help pay for the upgrades, but that pot of money will not cover all the costs. BART will need a bond, a rate increase, extra state and federal funding, increased revenue or a combination of the above to pay the bill.

But that decision is way down the road, Linton said.

“We’re looking at all options,” Linton said of the funding. “Now they’re going to be putting together final plans on what rail cars will need, and the government affairs side is looking how to fund it.”

(Click here if you are unable to view this photo gallery on your mobile device) The Ruth Bancroft Garden in Walnut Creek celebrates the life of its founder Ruth Bancroft who died at 109 on November 26, 2017. The Ruth Bancroft Garden is a nonprofit public dry garden that was planted by Mrs. Ruth Bancroft in 1972 and was opened to the...